In drilling operations, such as the drilling of hydrocarbon wells, drilled bores are usually lined with steel tubing known as casing, which is cemented in place by pumping cement into an annulus between the casing and the bore wall. Once a length of casing is in place, this imposes a restriction on the diameter of any subsequent section of bore, as the drill bit and any further casing must pass through the existing casing. However, reductions in bore diameter are undesirable as they tend to limit the production flow rate of hydrocarbons through the bore. Thus under-reamers are used to enlarge such subsequent sections of bore. Examples of under-reamers are disclosed in U.S. Pat. Nos. 6,378,632, 6,615,933, 4,589,504 and 3,712,854. Generally an under-reamer is used in a BHA up-hole of a drill bit. In this way the drill bit drills the borehole to be under-reamed at the same time that the under-reamer enlarges the borehole formed by the bit.
However, a problem with BHAs of this type is that if the under-reamer drills material which is much harder than that being drilled by the bit, excessive weight can be applied to the under-reamer. This can lead to premature wear and cutter damage. Also, in some circumstances, the opposite problem may occur, i.e. when the bit drills material much harder than that being drilled by the under-reamer, excessive weight can be applied to the bit.
Another problem is that as the material being drilled by the bit and under-reamer changes in hardness, the weight and torque can be transferred very rapidly between the two, leading to axial and rotational shocks. These shocks may be damaging to down-hole equipment.
In U.S. Pat. No. 5,343,964 a downhole tools is described where a central drill bit is connected with a coaxial drill bit. The central drill bit is driven by a downhole motor, and a circumferential drill bit is driven by drillstring rotation from the surface. The two bits are connected by an axial spring above the downhole motor and a prismatic connection below the motor that connects the stator of the downhole motor to the drillstring.
The most common need for a circumferential bit in well construction is to under-ream a hole to a size larger than the casing above the newly created hole. This necessitates a so-called ‘on-demand’ under-reamer, where the cutting elements are extended outward to their full diameter when required (normally after the under-reamer has cleared the casing shoe). The apparatus described in '964, is difficult to adapt to a use with an on-demand under-reamer, as the radial arrangement of central bit drive shaft, prismatic element and circumferential bit does not have the space to accommodate the under-reamer in its retracted position. Additionally one of the standard methods for under-reamer deployment, the use of a ball drop, would be impossible with the configuration as described.
It is therefore an object of the invention to provide an improved tool with applications in a wide range of under-reamer operations.